How do I disable Keychain to use LastPass for work

For assistance with creating this work-related configuration, contact your IT support group, and LastPass support. See how they want you to set this up.

There’s no “off” switch for Keychain, other than not using it.

I would usually set up a second login here though, and load and configure all of the IT stuff in that second login. Leave the main login and your own stuff alone. Unless this whole Mac is a work-provided computer, that is.

Check System Settings -> Passwords -> Password Options

Here you can tell the OS whether to use the Keychain for passwords and keys. Turning this off should do what you want.